For me this was caused by an issue with the ACF screen that was used during the REST requests missing the attachment key. Filtering the screen and adding for media posts worked for me
add_filter(‘acf/location/screen’, function($screen) {
if ($screen[‘post_id’]) {
$post = get_post($screen[‘post_id’]);
if($post && $post->post_type == ‘attachment’) {
$screen[‘attachment’] = $screen[‘post_id’];
}
}
return $screen;
}, 10, 1);
